It's a holder for Microphone, Hand, No.13 with a locking pressel switch, presumably for "hands free" use or long transmissions.
(Mic, Hand No.13 is the round bakelite "fist mike" that's part of the later British "Microphone & Receiver Headgear No.10" used with the WS19, 22, 52, 62, etc.)
Chris.
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